Process for the purification of epsilon-caprolactam



Dec. 23, 1969 H, HQFMANN ET AL 3,485,820

PROCESS FOR THE PURIFICATION oF -GAPROLACTAM Filed Aug. l5, 1966 3,485,820 PRQCESS FOR 'im PURlFlCATIQN OF e-CAPROLACTAM Hansdieter Hofmann, Petter-weil kreis Friedberg, Horst Corsepius, Frankfurt am Main, and Manfred Kyber, Bergen-Enkheim, Germany, assignors to Vickers-Zimmer Aktiengesellschaft Planung und Bau von Industrieanlagen, Frankfurt am Main, Germany Filed Ang. 15, 1966, Ser. No. 572,470 lint. Cl. C07d 41/06 US. Cl. 260-239.3 7 Claims ABSTRACT F THE DHSCLSURE A process for the purification of crude e-caprolactam by multiple recrystallization from organic semipolar or alkyl aromatic solvents in which the solubility of the e-caprolactam at 10 C. is less than 30 g. of the lactam per 100 g. of the solvent.

This invention relates to the purification of e-caprolactam used in the manufacture of Nylon 6 bers, particularly e-caprolactam prepared by ammonolysis.

lt is a known practice to purify crude e-caprolactam produced according to the Beckmann rearrangement of cyclohexanone oxime with subsequent extraction of the neutralized reaction solution by fractionate distillation of the evaporation residue or by recrystallization from solvents. This practice is said to achieve liber-grade quality lactam and good recovery yields. Solvents especially suitable are aliphatic hydrocarbons, such as, for example, n-heptane, cyclohexane, gasoline fractions and the like. Mixtures of said hydrocarbons with chlorinated hydrocarbons are likewise used for the purification of crude lactams.

Heretofore, water was also used for the purication of contaminated lactam.

Besides obtaining lactam by way of the Beckmann rearrangement of cyclohexanoneoxime, which is widely used commercially, other reactions, too, lead to e-caprolactam, such as, for example, hydrolysis of e-aminocapronitrile, reaction of cyclohexanone with hydrazoic acid (Schmidts reaction) and ammonolysis of e-caprolactone.

Because e-caprolactone is now readily available, the ammonolysis process has great prospect of technological realization. Especially favorable here is the fact that practically no ammonium sulfate is yielded, the disposal and further use of which is becoming more and more diicult.

However, these salt-free lactam processes are disadvantageous in that the crude lactam produced is much less pure than that produced yby the Beckmann rearrangement. The purication is rendered dicult, above all, by

the presence of 1,6-disubstituted, non-ring-form CG-compounds and lower lactams. These compounds are so similar in their chemical and physical behavior to that of e-caprolactam that they cannot satisfactorily be removed by the conventional lactam-purifying methods, such as extraction and rectification even in a combined manner of operation. These impurities impair the quality of iibers produced from the caprolactam. The solvents or solvent mixtures mentioned in the above-described recrystallization processes fail to purify satisfactorily lactams produced by ammonolysis.

nited States Patent O ICC We have found that crude lactam can be purified economically to products with fiber quality, if the crude lactam is subjected to a multi-stage recrystallization from solvents with specific solution properties.

The object of the invention is a process for the purifying of crude e-caprolactam by crystallization from one or more solvents with the characterizing feature that crude e-caprolactam is recrystallized two or more times from organic solvents in which the solubility of the ecaprolactam at 10 C. is less than 30 g. of the lactam per g. of solvent. The preferred organic solvents for the purpose of the invention include aliphatic monocarboxylic acid esters contains 5 to 11 carbon atoms, alkyl ketones containing 4 to 10 carbon atoms, and monoalkyl or dialkyl benzenes containing 8 to l0 carbon atoms.

The puriiication process for crude lactams according to the invention has proved especially favorable for crude lactams produced by ammonolysis of e-caprolactone and which contains as by-products predominantly e-hydroxycaproic acid nitrile, aminocaproic acid amide, e-aminocaproic acid nitrile, saponiiiable steam-volatile monocarboxylic acids, lower lactams and similar compounds that are ditiicult to remove. Preferably solvents are used which have semipolar properties and whose boiling points preferably lie below C. The recrystallization of the crude caprolactam is carried out with a ratio of solvent to lactam ranging from 025:1 to 2.5 :1. Especially preferred is a quantitative ratio of 0.75 :1 to 1.5 l.

The lactam purified according to the process of the invention can thereupon be subjected to a treatment with steam or nitrogen, which may be carried out in a vacuum in order to remove retained solvent traces.

For the achievement of a good yield in the purication and recovery from the lactam-containing mother liquor after separating 01T of the crystallizate of the first recrystallizing step and possibly after separating off of a second crystal fraction, a mixture of additional e-caprolactam and impurities is separated oli. The separation of this mixture from the solvent is accomplished either through extraction with water or by distilling off the solvent. The mother liquor residue is then returned to the ammonolysis. A further possibility of working up the mother liquor is that of precipitating the caprolactam present as the hydrochloride by introduction of dry hydrochloric acid. The resulting hydrochloride can, after separation, be decomposed in a known manner with diluted alkalies and be converted into lactam by extraction with solvents. There, expediently, the solvent from the multistage recrystallization process may be reused. At this point, the mother liquor remaining contains only slight amounts of lactam and can then be thrown out or can be subjected wholly or in part to ammonolysis into lactam, by itself or with fresh crude e-caprolactam. The solvents used are purified in a known manner by distillation or by extraction with water.

The execution of the puriiication according to the invention by multistage recrystallization can be carried out especially advantageously as a continuous counterflow crystallization.

The process of the invention has proved especially economical and favorable because it is possible to dispense with any chemical treatment, such as are described in the lactam purication processes of the prior art.

The following examples are to describe the process according to the invention for the purification of crude lactams, which come predominantly from the abovementioned salt-free lactam syntheses. Also regenerate lactam, pre-run lactam or lactam from distillation resiidues can be successfully recovered and purified by the method of the invention.

saturation. Thereupon, the crystalline deposit was drawn off on a glass frit suction filter. The deposit was decomposed in a known manner under cooling with diluted caustic solution and extracted exhaustively with benzene. After distilling off of the benzene, there remained 76.8 g.

of residue which contained 73.1% of lactam. EXAMPLE 1 The mother liquor can be freed of the dissolved hydro- A crude lactam coming from the amnionolySlS 0 f chloric acid by heating and introduction of nitrogen and icaprolactone and already concentrated had the following then appeal-s as a brown, Somewhat Viscous liquid whim composition: lo contains only traces of lactam.

. EXAMPLE 3 The residue is made up of high-boiling organic bylo products. The solidiiication point was 60 C. and the po- 250 parts of the crude lactam described in Example l tassium permanganate consumption amounted to ca. 50 were dissolved in heat in 200 parts of n-amylacetate .inn g./ 1000 g. of crude lactam, and the pH value of a crystallized at 0 C. After drawing off by suction, aftersolution Was 11.4. washing was done with 70 parts of solvent and there- A g. sample of the crude lactam was dissolved in 20 upon drying was carried out in a vacuum at 55 to 60 t1. l0 g. of each of the solvents listed in the table below and There were obtained 151.r parts of lactam with a solidiiibrought to crystallization under agitation at +5 to +10 cation point of 68.3 C. After the concentrating of the C. Then suction was applied in a'vacuum filter and aftermother liquor, there were obtained 31.2 parts more or washing was done with a little solvent. For the removal lactam. of the solvent, drying was carried out in a vacuum dryer The recovery yield up to this point amounted to 89.9%. at t0 60 C, After distilling off the solvent there remained a residue The solvents can be regarded as suitable which yield liquid at room temperature of 67.1 parts which still conwith maximal production of lactam the best quality of tained 30.5% of lactam. The liquid residue was returned lactam as .measured by the hardening point. in this form to the amnionlysis of the e-caprolactone` TABLE Solubility e- Lactamyield caprolaetani at with reference Hardening Evaluation 10 C. in g. pei' to the lactam point of the of tlie Solvent 100 g. solvent used, percent lactam, C. solvent Acetic acid ethyl ester 32 75.8 66.8 conditionally usable. Acetic acid n-butyl ester.. 1G 81.5 66.7 Good. Acetic acid sec. butylester. 22 79. 2 66.6 Do. n-Butyrie acid n-butyl ester. l1 84. 6 6b. b Do. Propionie acid isoamyl ester 10 82. 5 0 Do. Capronic acid ethyl ester 12 84. 1 66. 3 Do. Pelargonie acid ethyl ester--. 6 85.8 61.9 Do. Methyl isobutyl ketone 25 8l. 2 66. 7 Do. Di-isobutyl ketone 10 78. 6 66. 8 Do. 5-methyl liepanone-S.- 16 81. 6 61. 2 De. P-cymole 25 79. 9 66.7 Do. Cumole 16 80. 3 67. l Do.

The experiments clearly show the differences between Two further recrystallizations carried out in the same the classes of solvents. The best purification effect with manner with the above solvents led, after drying, to i maximal lactam yield is given by the higher esters, kelactam with the following characteristic numbers: Hardtones and alkyl aromatic substances. Acetic acid ethyl erung point +68.6 C. and potassium permanganate ester has the drawback that it has somewhat to@ great a number 2.000 sec. A polymerized and spun sample of the solubility for lactam and too high a volatility. These U0 lactam yielded a good result with completely normal properties stand in the way of its economical utilization. polymerization and spinning behavior.

The lactam yield in this series of tests ought to be greater EXAMPLE 4 than 79%. Further, it is apparent that with these impurities one recrystallization is no means Sucient The COl'l'tlnuOuS COUIICIHOW CTyStaHlZalOIl 1S CHITICLI 55 out according to the flow sheet shown in the accompany- EXAMPLE 2 ing drawing. 250 parts of crude lactam (composition or Exam le 2 wer d c 500 g. of a crude lactam obtained from ammonolysis lzed gh e .as escrltfed 1.n Example 3 @enigmi ree times in succession, in each case with tresn of e-caprolactone and concentrated, which contained t 8177 1 t d had a hadeninn Cim of 58o C was solvent. After this passage, the first recrystallization or 0 ac a.m an l a p t 60 another stream of crude lactam was carried out using the dissolved with heat in 500 ml. of sec.butyl acetate and h o mot er liquoi of the second recrystallization of the tirst crystallized under agitation at 0 C. Thereupon the cry- Stream Th e second recrystallization of the second stream stals were drawn off by suction onto a vacuum filter and was eected u sing the mother liquor of the third recrysafter-washed with 100 nil. of solvent. After drying, there tanizao f n rom the first stream. For the third recrystalliremained 264.7 g. of lactam with a hardening point of o 6., zation, from which the pure lactam emerges, fresh solvent 68.5 C. After concentration and through recrystalliza- W s used i h t, tion there were additionally obtained 83 g of a second a n eac s mgm' A o From the mother liquor of the first recrystallization ot crystal fraction with a solidication point of 68.1 C. each stream throu h g condensation and recrystallization This corresponds to a yield of 86.6% of the lactam h es t wh'ch fter re cated rec stallizafion u der the t ere Wa-S Obtained a Second Crystal fractlon which, i0- PT @T d tlorsah d a S12) dcati Org oint 'f 68 7E C and 70 gethei' with the crude lactam, was returned to the first same on i 21 b 1f 200 P 0 recrystallization. The solvent recovered in the concentraa Permangaafe num er 0 s e tion of the mother liquor was used again in the third For thefecovefy O the femalmflg lmouflt 0f lactam recrystallzation. Alternatively, the mother liquor obtiler? Was introduced mw the mQhfTlKllmf 0f feclfysfal' tained after recovery of this second crystal fraction can, lization, dry gaseous hydrochloric acid under cooling to as described in Example 2, be freed of residual lactam by introduction of hydrochloric acid, Other means of lactam recovery are the extraction of the lactam and byproducts with water, or evaporation of the solvent and return of the whole mother liquor residue into the ammonolysis.

The e-caprolactam recovered from the mother liquor in the form of lactam hydrochloride is likewise fed with the crude lactam back in the first recrystallization.

For the removal of the last solvent traces which are often stubbornly retained, the purified lactam, after the third recrystallization, was subjected to a vacuum treatment in a nitrogen atmosphere at temperatures around 100 C.

According to the counterflow recrystallization process described, there were used altogether 250 parts by weight of crude lactam according to Example 2.

Material balance (for tive passages) Raw material: Crude lactam 1250 parts, containing 1015 parts of lactam.

Parts Pure lactam obtained 837 lactam-containing by-products, mother liquor residue from the after-crystallization (containing 46.8%

of lactam) 378 Corresponding to a lactam amount of 177 The solidification points lay at 68.6 and 68.7 C.

The PM number iluctuated between 1000 and 3000. A polymerizate produced from the pure lactam exhibited excellent polymerization behavior and spinnability.

EXAMPLE 5 The rst runnings from a continuous lactam distillation, in which recovered crude lactam from the Beckmann rearrangement was used, had the following properties:

Solidiiication point 65.8 C. Lactam content 98-99 Potassium permanganate consumption 1.7 g./ 1000 g. of lactam.

100 parts of these first runnings were brought into solution by heating with 100 parts of n-butylacetate. By cooling to 0 C. crystallization was carried out and the crystal suspension produced was sharply drawn off on a suction filter. The crystals were washed with parts 0f solvent. After drying, 69.5 g. of E-caprolactarn were obtained with a soliditication point of 68.4 C. and a PM number of 300 sec.

Prom the mother liquor there were additionally obtained through concentration 15.6 g. of a second crystal fraction also having a solidication point of 68.4 C.

50 parts of the first crystallizate were subjected in like manner with 50 parts of n-butyl acetate to a further recrystallization. After filtration and washing there were obtained on a centrifuge, after drying, 35.5 parts of ecaprolactam, which had a solidiiication point of 68.6 C. and a PM number of 1300 sec.

The mother liquor of the second recrystallization can be used again successfully, as in Example 4, for the first recrystallization or there can be recovered by evaporation a further crystal fraction.

EXAMPLE 6 A residue from continuous lactam distillation exhibited the following properties:

6 Appearance Dark brown, semicrystalline. Lactam content ca. 45%. Potassium permanganate consumption ca. 30 g./ 1000 g. residue. Dimer lactam ca. 15%. Base number 9.3.

This residue was dissolved in a little water, treated with ammonium sulfate and exhaustively extracted with benzene.

After evaporation of the extract there remained a lactam residue with a solidification point of 64.6 C. This lactam residue was recrystallized three times in succession with n-butyl acetate in the ratio of 1:1, according to Example 5. After each recrystallization, the most important features of the e-caprolactam, solidiiication point and the PM number, were determined. (See table below.)

TABLE SP, C. PM No. sec.

Crystallizate from 1st recrystallization 68.0 25 Crystallizate from 2nd recrystallization. 68. 5 60 Ciystallizate from 3rd reerystallization 68. 6 200 By the recrystallization according to the invention, it is possible to produce a usable e-caprolactam even from residual lactam of very poor quality.

What is claimed is:

1. Process for the purification of crude e-caprolactam produced by ammonolysis of e-caprolactone by crystallization from solvent, characterized in that crude e-caprolactam is recrystallized at least twice in a continuous multistage counterflow crystallization from organic semipolar solvents selected from the class of aliphatic monocarboxylic acid esters containing 5 to 11 carbon atoms and aliphatic ketones containing 4 to 10 carbon atoms.

2. Process according to claim 1 in which said organic solvents are aliphatic monocarboxylic acid esters containing 5 to 1l carbon atoms.

3. Process according to claim 1, in which said organic solvents are aliphatic ketones containing 4 to 10 carbon atoms.

4. Process according to claim l, in which said crude e-caprolactam is the product of ammonolysis of e-caprolactone and the mother liquor of recrystallization is subjected to treatment to remove additional caprolactam.

S. Process according to claim 4, in which said mother liquor is treated with dry hydrochloric acid to precipitate caprolactam hydrochloride, the hydrocloride is decomposed to provide caprolactam and the caprolactam removed by solvent extraction.

6. Process according to claim 4, in which the mixture of caprolactam and impurities contained in the mother liquor is recovered and subjected anew to ammonolysis.

7. Process according to claim 1 wherein the ratio of solvent to crude lactam is about 0.2.5:1 to 2.521.

References Cited UNITED STATES PATENTS 2,313,026 3/1943 Schlack 260-239-3 2,813,858 11/1957 Joris S260-239.3 3,359,260 12/1967 Roberts et al. 260--239.3

HENRY R. l ILES, Primary Examiner R. T. BOND, Assistant Examiner 

